61A Lecture 12 Project 2 due Thursday 10/10 @ 11:59pm Guerrilla - - PDF document

61A Lecture 12

Monday, September 30

Announcements

• Homework 3 due Tuesday 10/1 @ 11:59pm
• Optional Hog Contest due Thursday 10/3 @ 11:59pm
• Homework 4 due Tuesday 10/8 @ 11:59pm
• Project 2 due Thursday 10/10 @ 11:59pm
• Guerrilla Section 2 this Saturday 10/5 & Sunday 10/6 10am-1pm in Soda
• Topics: Data abstraction, sequences, non-local assignment
• Meet outside Soda 306

For Statements

(Demo)

Sequence Iteration

def count(s, value): total = 0 for element in s: if element == value: total = total + 1 return total Name bound in the first frame

• f the current environment

(not a new frame)

For Statement Execution Procedure

for <name> in <expression>: <suite> 1.Evaluate the header <expression>, which must yield an iterable value (a sequence). 2.For each element in that sequence, in order:

• A. Bind <name> to that element in the first frame of the current environment.
• B. Execute the <suite>.

Sequence Unpacking in For Statements

>>> pairs = ((1, 2), (2, 2), (2, 3), (4, 4)) >>> same_count = 0 >>> for x, y in pairs: if x == y: same_count = same_count + 1 >>> same_count 2 A sequence of fixed-length sequences A name for each element in a fixed-length sequence Each name is bound to a value, as in multiple assignment

Ranges

..., -5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5, ...

The Range Type

>>> tuple(range(-2, 2)) (-2, -1, 0, 1) >>> tuple(range(4)) (0, 1, 2, 3) A range is a sequence of consecutive integers.*

range(-2, 2) Length: ending value - starting value Element selection: starting value + index Tuple constructor With a 0 starting value (Demo)

Membership & Slicing

>>> digits = (1, 8, 2, 8) >>> 2 in digits True >>> 1828 not in digits True >>> digits[0:2] (1, 8) >>> digits[1:] (8, 2, 8) The Python sequence abstraction has two more behaviors! Membership. Slicing.

Slicing creates a new object

Lists

['Demo']

List Comprehensions

[<map exp> for <name> in <iter exp> if <filter exp>] Short version: [<map exp> for <name> in <iter exp>] A combined expression that evaluates to a list using this evaluation procedure:

• 1. Add a new frame extending the current frame.
• 2. Create an empty result list that is the value of the expression.
• 3. For each element in the iterable value of <iter exp>:
• A. Bind <name> to that element in the new frame from step 1.
• B. If <filter exp> evaluates to a true value, then add the value of <map exp> to

the result list.

Dictionaries

{'Dem': 0}

Limitations on Dictionaries

Dictionaries are unordered collections of key-value pairs. Dictionary keys do have two restrictions:

• A key of a dictionary cannot be an object of a mutable built-in type.
• Two keys cannot be equal. There can be at most one value for a given key.

This first restriction is tied to Python's underlying implementation of dictionaries. The second restriction is an intentional consequence of the dictionary abstraction. If you want to associate multiple values with a key, store them all in a sequence.

Identity and Equality

(Demo)